1. Field of the Invention
The invention relates generally to GPS clocks and more particularly to an indoor GPS clock using long integration periods for acquiring and tracking GPS signals lower than about −143 dBm and providing disciplined frequency and time standard signals.
2. Description of the Prior Art
Conventional GPS positioning receivers provide GPS-based time as a byproduct of the resolution of a three dimensional GPS-based location. Typically, this time is issued in the form of a data packet that identifies the GPS-based times of the pulses of a one pulse per second (1 PPS) output signal. However, the accuracy for such time is limited by the cycle period of the local reference oscillator in the GPS receiver and any uncalibrated electrical length that the signal travels before it is used. There are several GPS timing applications where these limitations are not acceptable. Therefore, GPS timing applications commonly require a higher frequency signal, for example 10 MHz, for use as an accurate time base for maintaining an internal time standard. In order to meet the needs of these applications, a special type of GPS receiver, termed a GPS clock, has been developed. GPS clocks use supplementary techniques, such as clock bias feedback, for providing an output signal having a frequency that closely tracks the frequency of the GPS signal.
In some cases accurate time and/or frequency are required at a user location that is inside of a building. It is well-known that GPS receivers do not function well within a building because the building attenuates the GPS signal to a level that is too low for acquisition and/or tracking. It is sometimes possible to avoid this problem by placing the GPS signal antenna outdoors or by a window with a clear sky view and then conducting an amplified antenna output signal or the frequency and time signal to where it is needed. However, there are certain circumstances where this is impractical.
A clear-view GPS signal available to a GPS antenna on earth from a GPS satellite is specified as −160 dBW (−130 dBm) by the GPS system specification ICD-GPS-200 Rev C published by Arinc Research Corporation of El Segundo, Calif. published 10 Oct. 1993 and last revised 11 Oct. 1999. Misra and Enge in “Global Positioning System” ISBN 0-9709544-0-9 page 288 also show −160 dBW (−130 dBm) as received power available to an isotropic antenna from a satellite at the zenith. Conventional GPS receivers acquire GPS signals as low as −130 dBm using a signal integration period matching the C/A PRN code epoch period of one millisecond. In addition, it is known by those skilled in the art that an integration period matching the GPS bit time of 20 milliseconds can be used for achieving a processing gain of 13 dB in order to acquire and/or track GPS signals as low as about −143 dBm.
The attenuation of the building will vary a great deal depending upon the type of building and the depth within the building. However, it is typically true that a building attenuates a GPS signal by more than 13 dB so that the GPS signal is lower than the signal acquisition that is achievable with a 20 millisecond integration. Recently, many workers have applied a great deal of energy to inventing techniques for receiving low level (that is more than 13 dB lower than open sky) signals. Unfortunately, to date these techniques have not been applied in an indoor GPS clock that is capable of providing the frequency and time accuracies that are required.
One of the problems that must be resolved for low GPS signal levels is the problem of GPS signal carrier tracking. Conventional GPS receivers employ closed loop carrier phase or carrier frequency feedback corrections for tracking a GPS signal. However, below 27 dB-Hz C/N0 (−143 dBm referred to the antenna) and 24 dB-Hz C/N0 (−146 dBm referred to the antenna) typical GPS carrier phase and frequency lock loops, respectively, are not able to lock reliably. Existing military type GPS receivers have used a technique of carrier aiding by Doppler calculations. However, techniques used in military receivers are not directly applicable to a high performance OPS clock because a military GPS receiver must operate in a high dynamic environment whereas a GPS clock is expected to operate in an environment that is stationary. Furthermore, the GPS clock must provide an accurate frequency standard signal as an output that tracks the carrier frequency of the GPS signal.